1. Field of the Invention
The present invention relates to substrates comprised of a reinforced polymeric material overcoated with metal, and, more especially, to such substrates adapted for producing printed circuits. The topic metallized substrates are typically generically designated as "metal-clad".
2. Description of the Prior Art
Such metallized substrates are well known to those skilled in this art (compare U.S. Pat. No. 4,110,147). They generally comprise an electrically insulating support material and a conducting metal foil adhering to one or both of its face surfaces. This metal foil can be, in particular, a copper, aluminum, nickel or stainless steel foil having a thickness of between 10 and 100.mu., depending upon the type of printed circuit desired to be produced.
The metallized substrates in question can be rigid, semi-rigid or flexible, depending upon the composition of the insulating support material. The expression "semi-rigid substrate" designates a material which can withstand elastic deformation, by bending, down to a very small radius of curvature.
In the case of the rigid or semi-rigid metallized substrates, to which the present invention relates more particularly, the insulating support material is typically formed by stacking together a certain number of prepregs which each result from the association, known per se, of a reinforcing filler of elongate structure with a polymeric material. In the case of a common reinforcing filler, such as, for example, a glass fabric weighing 200 g/m.sup.2, an average of about 12 prepregs are used. The usual prepregs are comprised of cellulose papers, cotton fabrics or glass fabrics impregnated with synthetic polymers. Phenol/formaldehyde resins, polyester resins and especially polyepoxy resins are the products most frequently employed. The reinforcing filler, namely, paper or glass fabric, is generally impregnated with a solution of polymer in an appropriate solvent, and this enables the polymeric binder to penetrate thoroughly between the fibers of the filler. The impregnated structure is then passed through an oven heated to a temperature which enables the solvent to evaporate therefrom.
The manufacture of the metallized substrates consists of placing the stack of prepregs, covered with a metal foil on one or both of its face surfaces, depending upon whether it is desired to obtain a monometallized or bimetallized substrate, between the platens of a press. The stack is then compressed at a temperature which permits the association or consolidation of the various constituents. In certain cases, it is necessary to use an adhesive in order for the metal foils to adhere permanently to the prepregs.